Anti-platelet therapy with clopidogrel (Plavix) and aspirin is the standard of care for secondary prevention of myocardial Infarction. Despite its widespread use, 4 - 32% of Individuals are not responsive to clopidogrel. This renewal application will build upon significant progress made during the initial funding period in which we completed the Amish Pharmacogenomics of Anti-platelet lnterventlon-1 (PAPI-1) Study. Through the first genome-wide association study (GWAS) of its kind, we found that the loss of function cytochrome P450 2C19*2 (CYP2C19*2) variant is a major determinant of clopidogrel response, accounting for 12% of the variation in response. In an Independent cohort, we found that ~30% of the general population harboring CYP2C19*2 have poorer platelet response to clopidogrel and are at a 2.4-fold higher risk of having an ischemic cardiac event or death. The overall goal of this renewal application is to continue to advance the science of anti-platelet pharmacogenomics and its clinical translation. We hypothesize (a) CYP2C19 genotype-directed anti-platelet therapy will be superior to standard of care therapy; and (b) the genetic architecture of clopidogrel response Includes common and rare variants in yet-to-be identified genes. We have amassed a team of multidisciplinary investigators and collaborators and will capitalize on synergies created by active participation in the Pharmacogenomics Research Network to address the following Specific Alms: (1) To conduct the PAPI-2 Study, a prospective multicenter randomized double-blind clinical trial comparing cardiovascular events using CYP2C19 genotype-directed versus standard of care anti-platelet therapy in over 2000 patients with coronary heart disease; (2) To identify common variants in novel genes and loci for clopidogrel response by performing a large GWAS as part of a new Clopidogrel Pharmacogenomics GWAS Consortium; and (3) To identify rare variants in genes previously not known to influence platelet function or clopidogrel response by performing genome-wide exon (exome) sequencing from the extremes of the distribution of clopidogrel response. RELEVANCE: The proposed randomized clinical trial will provide the evidence base for translation of genotype-directed anti-platelet therapy into clinical practice. The Identification of common and rare variants in novel genes for clopidogrel response will provide new insights into platelet biology and variation in anti-platelet therapy response, and potentially, new targets for more effective agents to prevent and treat CHD.